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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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456                               ALASKAN GLACIER STUDIES
years, for the loess deposits overlying the outwash gravels immediately north of Wood Canyon have been at least 1000 years, and probably much longer, in process of accumulation.1
The Canyon Between Tasnuna River and Childs Glacier. The middle portion of Copper River Canyon extends from Tasnuna River to Childs Glacier (PI. CLXXIX). The width in this section is from £ to 4 miles, in contrast to the width of a half mile to a mile in the narrower canyon between Tasnuna River and Chitina. The width in this glacial canyon does not increase down-stream as in a valley carved by river erosion. Just below Tasnuna River, for example, the canyon is over four miles wide, and fourteen miles to the south at Allen Glacier it is four miles wide, while six miles farther down-stream the width is only two and one-half miles, and six miles below this, at Childs Glacier, the width is again four miles. The canyon walls are slightly lower than in the narrow, northern section, the average height in this middle section being 5000 to 6000 feet, attained within 1 to 2 miles of the river. The U-shape is well developed although the canyon walls flare more than in the northern section.
The broadened valley bottom of the Copper River canyon was recognized by Allen in 1885 as an evidence of profound glacial erosion.2
The cause for the increased width of the canyon from Tasnuna River to Childs Glacier seems to be the incoming of the Tasnuna and Bremner valleys. The Tasnuna, about two miles wide, and the Bremner, about a mile and a half in width, join the Copper, which is less than a mile wide, causing the four miles of width in the section of the Copper under discussion. This matter of width is another argument in favor of the recency of the diversion of the drainage of the Copper River basin across the Chugach Mountains. The relationship of widths of the Copper and these two tributaries is a natural one, granting profound glacial erosion by the Copper River Glacier, aided by the large former ice tongue in the Tasnuna. valley, which still contains the Woodworth and Schwan Glaciers, and the large, former ice tongue of the Bremner valley, where there are great glaciers even now.
The canyon walls between Tasnuna River and Childs Glacier are exceptionally simple, chiefly as a result of glacial erosion. There are no projecting spurs (PI. CLXXIX), except the tiniest remnants of eroded spurs, as in the one in the rock cut between Allen Glacier and Camp 55 at Abercrombie Rapids. The lower slopes of the canyon are much oversteepened, the upper limit of glaciation being estimated at between 1500 and 2000 feet north of Allen Glacier and 1450 feet at Childs Glacier. Below the shoulder of oversteepening the slopes are rounded and there is little post-glacial stream dissection.
Above the shoulder of oversteepening (a-a in PL CLXXXI, A,) there are many stream courses, separated by rather finely dissected spurs. The peaks include many pointed horns and broad towers, especially in the horizontal strata just north of Allen Glacier and Wernicke River.
The great predominance of striation and glacial rounding and polishing upon the lower slopes of the canyon emphasizes the few surfaces of bare rock which are hackly and innocent of glacial grooving. Among the places where these are found are^ (1) the pre-
. »Tarr, R. S. and Martin, La-wrence, Glacial Deposits of the Continental Type in ^aska, Journ. Geol., Vol. XXI, 1913, pp. 289-300. i Allen, H. T., Copper River, Alaska, Glacial Action, Science, Vol. VEX 1886, pp. 145-146.